This specification relates primarily to filtering flowable substances including polymers and to valves for use therein. Nevertheless, aspects of the features disclosed will be of use in other contexts.
Filtering of fluid thermoplastic polymers such as at least partially molten polyethylene, polypropylene and polystyrene, is an important process in polymer converting and scrap reprocessing where contaminants such as for example concrete dust or metal swarf have to be often removed. However, filters used in such applications can become rapidly clogged with the contaminants. It is extremely desirable that processing be continuous but the need to deal with contaminated filters poses problems.
In GB-A-1181075 there is disclosed an extended filter which can be advanced through inlet and outlet ports to introduce fresh filter parts into a filtering passage as required. In this arrangement, the ports are sealed by solid or semi-solid plugs. However, in cases of highly contaminated polymer this can become an expensive process as the filter will need to be advanced at very frequent intervals. Allowing the filter to become excessively clogged will not only reduce filtering efficiency but will also make advancing of the filter a difficult matter as there will be high frictional drag between the filter and a backing support plate that is typically used. If inexpensive filters are used, in an attempt to compensate for the costs of high filter consumption, these may break under the force applied to advance them.
In GB-A-1433867, there is disclosed a modified arrangement with two filters in a single passage which obviates the need for a stationary support. This deals with the frictional problems but filter consumption will still be high, and/or filtering efficiency reduced, in cases of highly contaminated polymer.
There has been proposed a system utilizing two filtering passages and two fixed filters. An example is the "Kleen Screen" (Trade Mark) system of Welding Engineers Inc. When one filter becomes clogged, the supply of polymer to its passage inlet is terminated. A valve which communicates with the passage between the inlet and the filter is opened, and the pressure of polymer which is still being filtered through the other passage causes a counterflow through the filter and out via the valve. The valve is then closed and flow to the passage is started again. When desired the same series of operations is carried out in the other passage, so that the filters are alternately cleaned by a counterflow. In cases of heavy contamination, however, the filters will rapidly become too clogged for counterflow cleaning to be sufficiently effective. This requires frequent laborious and time consuming manual replacement of an entire filter.